Furthermore, we are all familiar with the opposite effects of disuse.  Paralysis of an arm results in the cessation of its growth.  When a fall has injured the muscles and nerves of a child’s limb, that structure may fail to keep pace with the growth of the other parts of the body as a result of its disuse.  These are simple examples of a wide range of phenomena exhibited everywhere by animals and even by the human organism, demonstrating the plasticity of the organic mechanism and its modification by functional primary factors of variation.

But by far the greater number of variations seem to be due to the so-called congenital causes, which are sharply contrasted with the influences of the first and second classes.  It is quite true that the influences of the third class cannot be surely and directly demonstrated like the others, but however remote and vague they themselves may appear to be, their effects are obvious and real, while at the same time their effects are to be clearly distinguished from the products of the other two kinds.  Congenital factors reside in the physical heritage of an organism, and their results are often evident before an individual is subjected to environmental influences and before it begins to use its various organs.  For example, it is a matter of common observation that a child with light hair and blue eyes may have dark-eyed and brown-haired parents.  The fact of difference is a phenomenon of variation; the causes for this fact cannot be found in any other category than that comprising the hereditary and congenital influences of parent upon offspring. How the effect is produced by such causes is less important in the present connection than the natural fact of congenital variation.  Science, however, has learned much about the causes in question, as we shall see at a later point.

Thus the first step which is necessary for an evolution and transformation of organic mechanisms proves to be entirely natural when we give only passing attention to certain obvious phenomena of life.  The fact of “becoming different” cannot be questioned without indicting our powers of observation, and we must believe in it on account of its reality, even though the ultimate analysis of the way variations of different kinds are produced remains for the future.

Having learned that animals are able to change in various ways, the next question is whether variations can be transmitted to future generations through the operation of secondary factors.  Long ago Buffon held that the direct effects of the environment are immediately heritable, although the mode of this inheritance was not described; it was simply assumed and taken for granted.  Thus the darker color of the skin of tropical human races would be viewed by Buffon as the cumulative result of the sun’s direct effects.  Lamarck laid greater stress upon the indirect or functional variations due to the factors of use and disuse, and he also assumed as self-evident